Follistatin-Like Protein-1 (FSTL-1; also known as FRP or TSC-36) is an extracellular glycoprotein belonging to the BM-40/SPARC/osteonectin family of proteins containing both extracellular calcium-binding and follistatin-like domains. See e.g., U.S. Pat. No. 6,410,232. FSTL-1 was originally cloned from an osteoblastic cell line as a TGF-.beta. inducible gene. M. Shibanuma et al., Eur J Biochem 217, 13 (1993). The protein occurs in two isoforms resulting from differential sialylation. FSTL-1 has been detected in the medium of all osteosarcoma and chondrosarcoma cell lines, and in some cells of the fibroblast lineage. In mice, the highest expression of FSTL-1 has been observed in the lung. J. Mashimo et al., Cancer Lett 113, 213 (1997).
The action of FSTL-1 is unclear, and both proliferative and anti-proliferative effects have been reported. It is thought that FRP may play a role in neuralization during embryogenesis and its expression is upregulated by estrogen. See K. Okabayashi et al., Biochem Biophys Res Commun 254, 42 (Jan. 8, 1999) and T. Ohashi et al., Calcif Tissue Int 61, 400 (November, 1997). In contrast to other BM-40 family members, the extracellular calcium-binding domain of FSTL-1 is non-functional, suggesting that, despite its sequence homology to BM-40, it has evolved clearly distinct properties. H. O. Hambrock et al., Journal of Biological Chemistry 279, 11727 (Mar. 19, 2004). Analysis of prostate cancers has revealed that over-expression of FSTL-1 may be associated with higher metastatic potential. L. Trojan et al., Anticancer Res 25, 183 (January-February, 2005). In contrast, FSTL-1 expression has been extinguished in v-ras-transformed rat fibroblasts, and transfection of FSTL-1 into these cells inhibited in vitro invasion and led to growth inhibition in human lung cancer cells. See I. M. Johnston et al., Oncogene 19, 5348 (Nov. 9, 2000) and K. Sumitomo et al., Cancer Lett 155, 37 (Jul. 3, 2000).
In addition, it has previously been shown that FSTL-1 is highly-upregulated in the joints during the acute phase of collagen-induced arthritis (CIA), most prominently at the junction of synovium and eroding bone, suggesting a role in joint destruction. S. Thornton et al., Clin Immunol 105, 155 (2002).
In 1998, Tanaka et al. cloned FRP from rheumatoid arthritis (RA) synovial tissue and demonstrated the presence of anti-FSTL-1 antibodies in the serum and synovial fluid of RA patients. M. Tanaka et al., International Immunology 10, 1305 (1998). In addition, Tanaka et al. analyzed the mRNA expression and protein expression of FRP in from patients with RA and patients with osteoarthritis (OA) and found that the FRP mRNA expression was higher in RA than in OA synovial samples. Importantly, Tanaka et al. concluded that there was no difference in the protein levels of FRP between these two groups.
Ehara et al. measured mRNA expression of FRP in synovial fluid from patients with RA and patients with OA. They found the mRNA expression of FRP was 2.3 fold higher in the RA patients than in the OA patients. Y. Ehara et al., Clin Exp Rheumatol 22, 707 (2004). Importantly, the authors stated that the FRP may exert a protective effect for joint destruction on synoviocytes.
Other groups have used mass spectroscopy to characterize the expression of a large number of genes to determine whether one or a combination of genes could be used for diagnostic purposes. For example, in WO 2005/032328, over 500 genes are disclosed as part of a mass screening. One of these genes (M285 in Table 1) is FSTL-1. Importantly, the data shows that the levels of FSTL-1 protein decreases in patients with erosive arthritis and also in patients with non-erosive arthritis as compared to healthy (normal) individuals (i.e., without arthritis).
Another publication, WO 2004/0018522 describes measurement of mRNA expression levels for a large number of genes to diagnose or predict multiple sclerosis. FSTL-1 appears in Table 3 and 9, however, only mRNA levels are measured and there is very limited disclosure that connects such expression in multiple sclerosis patients with arthritis (or, by extension, to other rheumatic diseases).
Finally, Miyamae et al. reported that FSTL-1 was a novel pro-inflammatory molecule with an unrecognized role in inflammation. T. Miyamae et al. J. Immunol. 177, 4758 (2006). Importantly, there was no disclosure in this reference that teaches or even suggests using protein levels of FSTL-1 as a biomarker for assessing disease severity in inflammatory diseases. As such, the invention describes a novel diagnostic biomarker that is capable of assessing disease severity in inflammatory and/or rheumatic diseases.